1. Field
This invention relates generally to a pulsed fiber laser amplifier system and, more particularly, to a pulsed fiber laser amplifier system that employs a plurality of optical sources providing seed pulse beams at different wavelengths and a single fiber amplifier chain that amplifies all of the seed pulse beams at different time intervals, where the spaced apart amplified seed pulse beams are combined into a single pulse output beam using a spectral-temporal beam combiner.
2. Discussion
High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these applications. One known type of laser amplifier is a fiber laser amplifier that employs a doped fiber and a pump beam to generate the laser beam, where the fiber has an active core diameter of about 10-20 μm or larger.
Improvements in fiber laser amplifier designs have increased the output power of the fiber to approach its theoretical power and beam quality limit. To further increase the output power of a fiber amplifier some fiber laser systems employ multiple fiber lasers that combine the fiber beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fibers in a coherent manner so that the beams provide a single beam output having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the beam quality of the beam, where the more the more uniform the combined phase front the better the beam quality.
In one known multiple fiber amplifier design, a master oscillator (MO) generates a signal beam that is split into a plurality of fiber beams each having a common wavelength where each fiber beam is amplified. The amplified fiber beams are then collimated and directed to a diffractive optical element (DOE) that combines the coherent fiber beams into a signal output beam. The DOE has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different angular direction are redirected by the periodic structure all of the beams diffract from the DOE in the same direction. Each fiber beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the fiber beams is maintained coherent. However, limitations on bandwidth and phasing errors limits the number of fiber beams that can be coherently combined, thus limiting the output power of the laser.
To overcome these limitations and further increase the laser power, multiple master oscillators are provided to generate signal beams at different wavelengths, where each of the individual wavelength signal beams are split into a number of fiber beams and where each group of fiber beams has the same wavelength and are mutually coherent. Each group of the coherent fiber beams at a respective wavelength are first coherently combined by a DOE, and then each group of coherently combined beams are directed to a spectral beam combination (SBC) grating at slightly different angles that diffracts the beams in the same direction as a single combined beam of multiple wavelengths. The SBC grating also includes a periodic structure for combining the beams at the different wavelengths.
One specific application for fiber laser amplifiers is for 3-D Ladar range finding of objects that may be at a considerable distance. Laser beam pulses emitted by the fiber laser amplifier are reflected off of the object being targeted, and the reflected pulses from the object are collected by a receiver that includes, for example, an avalanche diode charge coupled device (CCD) array that provides both temporal and spatial imaging of the object to provide range information. By employing pulse widths of about 1 ns, images of the object can be achieved with a spatial resolution of inches at a range of 100 kilometers or more, thus providing a usable system for both terrestrial and space applications.
For certain fiber laser range finder systems, the fiber amplifiers operate in the <200 kHz range, where the energy storage capability and hence the average power capability of the fiber is limited because of peak power limitations. Typical large core fibers, i.e., 40 μm, often employed for these applications can store energies of about 2.5 mJ/pulse, but are limited to about 250 μJ/pulse at 1 ns pulse width by nonlinearities, such as self-phase modulation, in the fiber, which broadens the beam spectrum beyond what is usable for many applications. Further, these applications often require electrically efficient compact laser sources in the 200 W average power range. Such amplifier systems require pulse energies of 5 mJ/pulse, 0.5-1 ns pulse widths, and are limited to repetition rates of less than 40 kHz by a single photon counter detector readout electronic rate and require laser spectral bandwidths less than 1 nm so that the receiver can discriminate against solar background effects.
The pulse energies referred to above for laser range finding applications cannot be achieved using a single fiber amplifier chain because of the material limitations of the fiber. In order to obtain the desired power for these applications a number of fiber amplifier chains operating at different wavelengths are typically employed, where the fiber beam from each fiber amplifier chain is combined by, for example, a spectral beam combiner to provide a single pulse overlapped output beam at the desired power level. However, providing many fiber chains to provide the desired power significantly increases the complexity of the system, the cost of the system and the ability to package the system in a reasonable manner.